In particular, the invention has application in the hot dip galvanising industry which uses ammonium chloride and/or zinc ammonium chloride solutions as pre-flux baths for the preparation of the surface of the steel for galvanising. Prior to the fluxing process, the steel is subject to a pickling process and the steel being carried from the pickling bath to the pre-flux bath will often carry some pickle solution with it. This leads to contamination of the pre-flux bath with pickle liquor which causes the accumulation of iron in the pre-flux bath in the form of dissolved ferrous ion. In order to maintain the quality of the pre-flux solution the iron must be periodically or continuously removed. The standard industry method is treatment with hydrogen peroxide to oxidise the ferrous ion no the ferric ion together with pH control by ammonia addition to precipitate the iron as ferric hydroxide. This is usually done on a batch basis and the ferric hydroxide precipitate removed as a sludge after one or two days settling and subsequent decantation of the clear liquor.
This method suffers from considerable waste of solution due to entrainment in the sludge. The pre-flux bath is also subject to variation in composition and quality of the solution with time between treatments and there is a considerable bath down time for each precipitation treatment. Furthermore, the messy sludge that is produced must be dealt with and there is difficulty in disposing of the impure iron hydroxide as well as the inherent dangers o f handling a dangerous chemical such as a strong hydrogen peroxide solution. One means of reducing the contamination of the pre-flux bath used to date has been to provide a rinse bath between the pickling and pre-flux dips. This is only a partial solution, however as the disposal problems associated with the contaminated rinse water must be dealt with and any rise in iron level in the pre-flux solution cannot be directly remedied.
In German Patent No. 3630157 it is proposed to treat contaminated pre-flux liquor electrolytically in a separate vessel. The treatment comprises applying a direct current to a simple electrolytic cell having a pair of plate electrodes, the cathode being made of iron or aluminium and the anode made of graphite, positioned in an open bath of electrolyte (in this case the contaminated pre-flux liquor) to precipitate ferric hydroxide. The efficiency of a cell of this kind is quite low, particularly when the current density exceeds that required for iron oxidation at the anode. In operation, the precipitate is continuously removed from the cell by a liquid current flow which sweeps the precipitated mud into a filter device to prevent deposits of ferrous mud forming on the bottom of the flux vessel. The patent is primarily concerned with the provision of the filter device. The flow through the cell need only be sufficient to cause the ferric mud to become entrapped in the filter device and therefore iron transport to the anode is low. The German Patent requires an alternating current to be superimposed on the direct current to prevent dendritic deposits on the electrodes.
The present invention provides a process in which the turbulence is induced at, or in the proximity of, the anode to enhance mass transport to the anode with the result that a mechanically stable non-dendritic ferric hydroxide precipitate is grown on or near the anode. The precipitate on the anode may be progressively scoured from the anode and carried from the cell as a granular precipitate having good settling properties.